Process of coating



Patented Aug. 7, 1934 UNITED STATES.

PATENT OFFICE No 1mm. Application November 1:. 1932. Serial No. 643,539

Claims. (0!. 91-68) The invention relates to the coating of magnesium articles with decorative or corrosion-resistant films, such as paint, and is directedto the preparation of the magnesium surface to receive 5 and hold such a coating.

The readiness with whichpaint films peel away from untreated magnesium surfaces has been one of the chief difliculties in protecting this metal from corrosion. It is an objectof the present invention to obviate this and other diiliculties encountered in the coating of magnesium surfaces to protect them from corrosion or for the purpose of ornamentation. A special object is to provide a method ofpreparing the magnesium surface which is to be coated whereby the coating will be rendered more adherent. A further spe-. ciiic object is the provision of .a method of treating a magnesium surface so as to'increase the mechanical bond between such surface and any suitable applied coating thus greatly minimizing. if not altogether eliminating, the tendency of the applied coating to peel away from the surface.

These and other objects and advantages will more fully appear in the following description with reference to certain preferred processes falling within the scope of my invention.

According to my improved process, the surface of the magnesium article to be coatedlis treated in such a manner as to produce minute undercut 0 pits characterized in general by their small size. depth and even distribution. These undercut pits furnish a mechanical bond or "tooth" for priming coats, paints. lacquers. electroplatings.

and-in general all protective or decorative films which are adapted to flow into or be deposited in thepitsandthuskeythefllmtothemagnesium Most mineral and organic acids willattack and pit mesium alloys but the pits produced are smoothandahallow and therefore notadapted to keying a coating in position. Ammonium chloride solutions behave in much the same manner as acid solutions in attacking magnesium. Bolutions containing ammonium chloride and potas- 46' sium dichromate produce large and rather widely separated pits which, although they may be undercuttoacertainextent,aretoolargeandtoo widely spaced to be satisfactory.

I have found that the character of surface de- 60 sired may be easily and effectively produced by immersing the magnesium article in a solution of sodium chloride and thereafter treating the magnesium article with a solution of chromic acid. In selecting the concentrations of the solutions of sodium chloride, or other alkali metal or alkaconcentration and temperature are the variables which will control time of treatment. The concentrations of the solutions may be low and the temperature high if time permits or a longer treatment will result with decreased temperature. While best results are obtained using solutions 60 containing not less than Lper cent of sodium chloride and solutions containing 5 to 20 per cent of chromic acid, good results may be obtained with other concentrations, the time of treatment being varied according to the temperature employed; In the preferred practice of the invention I employ solutions at temperatures which may be generally designated as hot. i. e. above 50 centigrade and preferably between about 80 centigrade to boiling, but lower temperatures may be used.

Chlorides of other alkali metals or of the alkaline earth metals may be substituted for sodium chloride, although sodium chloride is to be preferred because of its cheapness and general availability. Thus agents such as calcium chloride, barium chloride, potassium chloride. etc.-, may be used in the first step of the treatmentand the effect produced will be substantially the same. The undercut pits formed during this step .of $5 the process are filled with a corrodon product probably consisting principally of magnesium hydroxide, magnesium oxy chloride. etc. The second step of the process, in which the corrosion product is removed by the chromic acid wash. leaves the pits clean and free to receive the coatins substance. The treatment which I have devised is thus essentially a two-step process or a three-step process according to whether it is considered as a method of preparing the magnesium as surface for coating or as a method of coating.

Quite obviousw there may be and preferably will be additional steps in the complete process such as cleaning the surface prior to the chloride treatment, washing between the chloride and chromic acid treatments. and washing and drying after the chromic acid treatment and before application of the coating substance. The preliminarycleaningmaybeperformedinanywell known manner such-as by sand blasting the sur face or by an acid dip. These steps are subordinate to those outlined in the two preceding paragraphs and may be regarded as a part of the operating technique incidental to a coating process or surface treatment of this As a specific example of my preferred process, the following treatment may be cited:

(1) The casting was sand blasted, and

(2) Immersed in a boiling 3 per cent solution of NaCl; duration of treatment, 4 minutes;

(3) Washed with water;

(4) Immersed in a boiling 20 per cent solution of Cl'Oa; duration of treatment, 2 minutes;

(5) Washed with water, and dried;

(6) Then painted.

The treatment is applicable to articles of magnesium and magnesium base alloys in general, and the term magnesium as used herein and in the appended claims denotes both magnesium and magnesium base alloys.

The specific terminology used herein is employed in order to enable those skilled in the art to readily practice my invention, and with full reliance on the doctrine of equivalents.

I claim:

l. The method of preparing the surface of magnesium articles for coating with paint and the like"which comprises subjecting the surface of the article to the action of a solution of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting the surface of the article to a chromic acid wash.

2. The method of preparing the surface of magnesium articles for coating with paint and.

the like which comprises subjecting the surface of the article to the action of a solution containing not less than about 1 per cent by weight of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting the surface of the article to the action of a 5 to 20 per cent solution by weight of chromic acid.

3. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjecting the surface of the article to the action of a hot solution containing not less than about 1 per cent by weight of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting the surface of the article to the action of a-boiling 5 to 20 per cent solution by weight of chromic acid.

4. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjecting the surface of the article to the action of a solution containing not less than about 1 per cent by weight of a chloride of a metal of the group consisting of the alkaline metals and alkaline earth metals, and thereafter subjecting the surface of the article to the action of a 5 to 20 per cent solution by weight of chromic acid, said solutions of metallic chlorides and chromic acid being maintained at a temperature of not less than about degrees centigrade.

5. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjecting the surface of the article to the action of a sodium chloride solution and thereafter subjecting the surface to a chromic acid wash.

6. The method of coating the surface of magnesium articles which comprises subjecting the surface of the article to the action of a solution of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals,

As a specific example of my preferred process, the following treatment may be cited:

(1) The casting was sand blasted, and

(2) Immersed in a boiling 3 per cent solution of NaCl; duration of treatment, 4 minutes;

(3) Washed with water;

(4) Immersed in a boiling 20 per cent solution of CrOa; duration of treatment, 2 minutes;

(5) Washed with water, and dried;

(6) Then painted.

The treatment is applicable to articles of magnesium and magnesium base alloys in general, and the term magnesium as used herein and in the appended claims denotes both magnesium and magnesium base alloys.

The specific terminology used herein is employed in order to enable those skilled in the art to readily practice my invention, and with full reliance on the doctrine of equivalents.

I claim:

1. The method of preparing the surface of magnesium articles for coating with paint and the like"which comprises subjecting the surface of the article to the action of a solution of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting the surface of the article to a chromic acid wash.

2. The method of preparing the surface of magnesium articles for coating with paint andv the like which comprises subjecting the surface of the article to the action of a solution containing not less than about 1 per cent by weight of a chloride of a metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting, the surface of the article to the action of a 5 to 20 per cent solution by weight of chromic acid.

Patent-No. 1,968,922.

3. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjecting the surface 'of the article to the actionof a hot solution-containing not less than about 1 per cent by weight of a chloride ofa metal of the group consisting of the alkali metals and alkaline earth metals, and thereafter subjecting the surface of the article to the action of a-boiling 5 to 20 per cent solution by weight of chromic acid.

4. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjectingthe surface of the article to the action of a solution containing not less than about 1 per cent by weight of a chloride of a metal of'the group consisting of the alkaline metals and alkaline earth metals, and thereafter subjecting the surface of the article to the action of a 5 to 20 per cent solution by weight of chromic acid, said solutions of metallic chlorides and chromic acid being maintained at a temperature of not less than about degrees centigrade.

5. The method of preparing the surface of magnesium articles for coating with paint and the like which comprises subjecting the surface of the article to the action of a sodium chloride solution and thereafter subjecting the surface to a chromic acid wash.

6. The method of coating the surface of magpesium articles which comprises subjecting the surface of the article to the action of a solution of a chloride ofa metal of the group consisting of the alkali metals and alkaline earth metals,

coating of paint.

ROBERT T. WOOD.

CERTIFICATE OF CORRECTION.

August 7, 1934.

ROBERT 1 W001).

- It is hereby certified that error appears'in the printed speciiicat ion 'of the above numbered patent requiring correctionas follows: Page 2, line 92, claim 4, for 'alkaline" read alkali; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this um a, of September, A. D. 1934.

Leslie Frazer Acting Conmissioner of Patents. 

